Electronic devices such as a personal computer, a cell phone, and the like include a switching power supply circuit (DC-DC converter) that supplies a driving voltage to internal circuits that perform signal processing. The switching power supply circuit converts a direct current voltage supplied from, for example, an AC adapter or a battery into a driving voltage suitable for operations of the internal circuits. For example, the switching power supply circuit controls a main switch to turn on and off and generates a direct current output voltage by stepping up or stepping down a direct current input voltage. Further, the switching power supply circuit performs a feedback control to maintain the direct current output voltage supplied to a load at a constant target voltage.
In recent years, a demand for reducing the size of the switching power supply circuit is increasing accompanying a widespread of portable type electronic devices such as a laptop type personal computer, a cell phone, and the like. In order to address such a demand, a single inductor multiple output type (Single Inductor Multiple Output: SIMO) DC-DC converter that may obtain a plurality of outputs from one inductor (coil) is being proposed. In such a type of DC-DC converter, the single inductor is shared to obtain the plurality of outputs. This may suppress an increase in the number of components, accompanying an increase in the number of the outputs, and an increase in a circuit area.
In the multiple output type DC-DC converter, a switching cycle corresponding to each output (load) is allotted beforehand, and power generated in each switching cycle is supplied to the corresponding load. For example, when there are two loads, the switching cycles are alternately allotted to the two loads. Further, in each of the switching cycles, a time during which the main switch is turned on (duty ratio) for causing a current corresponding to an input voltage to flow in the single inductor is adjusted depending on a magnitude of the corresponding load. In order to cause such a DC-DC converter to operate stably, it is preferable to cause a coil current flowing in the inductor to be zero by the time of an end of each switching cycle. That is, in order to cause the DC-DC converter to operate stably, it is preferable to operate in a discontinuous conduction mode (DCM) in which a change in a coil current IL between the switching cycles becomes discontinuous. This is because when the DC-DC converter is operated in a continuous conduction mode (CCM) in which the change in the coil current IL between the switching cycles becomes continuous, energy remaining in the inductor is discharged to another load in a subsequent switching cycle, and an output voltage becomes unstable. However, when the DC-DC converter is operated in the DCM, efficiency is lower than when operating in the CCM.
The single inductor multiple output type DC-DC converter operable in the CCM is described in U.S. Pat. No. 7,538,527, U.S. Pat. No. 7,312,538, U.S. Patent Application Publication No. 2008/0130331, and D. Trevisan et al, “Digital Control of Single-Inductor Multiple-Output Step-Down DC-DC Converters in CCM”, IEEE TRANSACTIONS ON INDUSTRAIL ELECTRONICS, Vol. 55, No. 9, September 2008, 3476-3483. FIG. 18 illustrates an example of an SIMO DC-DC converter. A DC-DC converter 6 illustrated in FIG. 18 is a step-down type DC-DC converter of a synchronously rectifying method that generates two output voltages Vo1 and Vo2 that are lower than an input voltage Vi based on the input voltage Vi.
As illustrated in FIG. 18, the DC-DC converter 6 includes a main switch SW11 to which the input voltage Vi is supplied, a synchronization switch SW12, and an inductor (coil) L11 coupled to a coupling node between the switches SW11 and SW12. Further, the DC-DC converter 6 includes output switches SW13 and SW14 coupled to the coil L11, and capacitors C21 and C22 respectively coupled to the output switches SW13 and SW14.
Further, the DC-DC converter 6 includes a circuit 111, which generates a feedback voltage VFB21 corresponding to a combined value of the two output voltages Vo1 and Vo2, and an error amplification circuit 112, which amplifies a voltage difference between the feedback voltage VFB21 and a reference voltage Vr1 to generate an error signal S11.
Further, the DC-DC converter 6 includes a PWM (Pulse Width Modulation) control circuit 113 that controls the switches SW11 and SW12 to turn on and off in a complementary manner based on the error signal S11. Further, the DC-DC converter 6 includes a circuit 114, which includes an error amplification circuit 115 that generates a signal S12 according to a voltage difference between the two output voltages Vo1 and Vo2, and a PWM control circuit 116, which controls the output switches SW13 and SW14 to turn on and off in a complementary manner based on the signal S12.
In this manner, the DC-DC converter 6 controls the input switches SW11 and SW12 based on the combined value of the two output voltages Vo1 and Vo2 and controls the output switches SW13 and SW14 based on the voltage difference between the two output voltages Vo1 and Vo2.
However, in the DC-DC converter 6, a voltage accuracy of the output voltages Vo1 and Vo2 is not satisfactory. This is because the output voltages Vo1 and Vo2 exist on two feedback loops. That is, the output voltages Vo1 and Vo2 are prone to influences of relative variations in all resistors and offset variations in the error amplification circuits 112 and 115. Thus, the voltage accuracy of the output voltages Vo1 and Vo2 is low.